µtvm debug improvements (#5648)

µtvm debug improvements (#5648)

* Forever loop in UTVMDone to aid debugging

* Use parameter and callback function as a micro debug hook.

 * Previously, users had to uncomment a region of code in
   micro_session.cc and recompile to debug. Now they can pass in a
   key in the micro.Session config:

       config = tvm.micro.device....generate_config()
       config['debug_func'] = _python_launch_gdb
       with micro.Session(config) as sess:
         ....

* clang-format

* Only forever loop on device (on host this blocks unittests)